That is pretty much how I imagined Thorin. I didn't imagine some lumpy ancient dude with white hair. I imagined a dark haired dwarf with some grey, more than they have on Armitage but not completely grey and I imagined the dwarves in general as more sharp featured, more like little stocky people but to be honest with Fili and Kili being the youngest, in my head, while they were both blond and bearded, I did tend to imagine them as being less stocky or "solid" looking than the older dwarves.

So in my case the changes to these characters really aren't a huge deal because I never imagined them looking like what we ended up with Gimli(who's great I have nothing against Gimli but IMO he actually does look older than I'd think he'd need to being a dwarf in the slightly lower end of middle age). I never imagined Thorin as being ancient and white or grey and I never imagined Fili and Kili as being carbon copies of the older dwarves, in my head they were always thinner for lack of a better word.

Yes they all look different than they did in my head but I still think they look great and have no problem with any of them really. I really think everything that looks in a still picture with no context like "Oh my god they've gone Hollywood!" is going to have a reasonable "in film" explanation which might possibly even heighten the emotion of what is going on.